During the drilling of a wellbore, various fluids are used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. During this circulation, a drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
The selection of the type of a wellbore fluid to be used in a drilling application involves a careful balance of both the good and bad characteristics of the wellbore fluids in the particular application and the type of well to be drilled. Frequently, the selection of a fluid may depend on the type of formation through which the well is being drilled. Wellbore fluid compositions may be water- or oil-based and may comprise weighting agents, surfactants, proppants, and polymers. However, for a wellbore fluid to perform its functions and allow wellbore operations to continue, the fluid may stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, practically all of the wellbore fluid may be lost to the formation. For example, wellbore fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole. Thus, fluid loss or lost circulation is a recurring drilling problem, characterized by loss of wellbore fluids into downhole formations that are fractured, highly permeable, porous, cavernous, or vugular or can be artificially induced by excessive mud pressures.
There is an analogous need to seal and prevent fluid loss when recovering hydrocarbons from sand formations, particularly depleted sand formations. Depleted sand formations are productive, or formerly productive, hydrocarbon zones that have been produced, drawn down, or otherwise depleted of their content, creating a lower formation pressure than that of the fluid which may be in use in the well. Because of this pressure differential, the sand formation may be partially or completely seal to inhibit or prevent fluid loss of the mud into the sand.
To combat such mud losses into the formation, lost circulation treatments are attempted to plug or block the openings either naturally formed or induced by the drilling operation. Such lost circulation treatments have included a variety of treatment materials, including polymeric based treatments having sufficient strength and integrity to minimize lost circulation into voids in direct communication with the wellbore, such as fractures, fracture networks, vugs, washouts, cavities, and the like. For example, crosslinkable or absorbing polymers, loss control material (LCM) pills, and cement squeezes have been employed. These additives have found utility in preventing mud loss, stabilizing and strengthening the wellbore, and in zonal isolation and water shutoff treatments. Some typical viscosifying additives used in well fluids to combat fluid loss include natural polymers and derivatives thereof such as xanthan gum and hydroxyethyl cellulose (HEC). In addition, a wide variety of polysaccharides and polysaccharide derivatives may be used, as is known in the art.
Further, providing effective fluid loss control without damaging formation permeability in completion operations has been a prime requirement for an ideal fluid loss-control pill. Conventional fluid loss control pills include a variety of polymers or resins, calcium carbonate, and graded salt fluid loss additives, which have been used with varying degrees of fluid loss control. These pills achieve their fluid loss control from the presence of specific solids that rely on filter-cake build up on the face of the formation to inhibit flow into and through the formation. However, these additive materials can cause severe damage to near-wellbore areas after their application. This damage may reduce production levels if the formation permeability is not restored to its original level. Further, at a suitable point in the completion operation, the filter cake may be removed to restore the formation's permeability to its original level.